Siri is great. The other day I was listening to music on the way home from the gym and I was able read and reply to text messages as I walked to the bus stop all without taking my iPhone out of my gym bag.
I wouldn't say Siri is reason alone to upgrade. For me that was the enhanced graphics of the A5 chip, the kick ass camera and the new 64 GB capacity. The fact I was upgrading from a second gen iPod touch and a $50 Nokia helped. Other's mileage may vary.